Washington is buzzing about a mysterious grand jury fight between Mueller's office and an unknown witness

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Robert Mueller.

• Washington is abuzz over a mystery case involving the special counsel Robert Mueller and an unnamed witness over a grand-jury subpoena.
• The case has been shrouded in secrecy since it was first mounted in August, and few details have trickled out since.
• This week, we learned that the unidentified witness in the case is not an individual, but a corporation from a foreign country.
• On Tuesday, the Washington, DC, Circuit Court of Appeals handed prosecutors a victory when it determined that the unnamed corporation has to comply with the grand-jury subpoena.
• "At the end of the day, these items will become known to a grand jury" and, potentially, the American public as well, said one DOJ veteran who's been following the case.

Fervent observers of the special counsel Robert Mueller's investigation into Russia's interference in the 2016 election have been captivated over the last few months by a protracted court battle between Mueller and an unknown party over a grand jury subpoena.

The case has been shrouded in secrecy since at least September, and few details have trickled out about it since.

• Mueller's office appears to have mounted the case - which is referred to in court documents as "Sealed v. Sealed" - on August 16.
• CNN spotted several lawyers from Mueller's office entering a courtroom at a Washington, DC, federal courthouse in September.
• They were arguing against an unknown defense team before Chief Judge Beryl Howell of the DC district court, who oversees federal grand-jury related cases.
• Politico also confirmed a link between the case and Mueller's team when a reporter working for the outlet overheard a man request a document in the case from Mueller's office. Politico reported that the man declined to identify himself when approached by the reporter.
• On September 19, a little over a month after the case was initially brought, Howell issued a ruling.
• Five days later, one of the parties in the case appealed Howell's ruling to the DC Circuit Court of Appeals.

Read more: A former federal prosecutor thinks Mueller's quiet period before the midterms may not have been so quiet after all

Nelson Cunningham, a former federal prosecutor and Senate and White House aide, wrote in Politico that the quick nature with which the case and the appeal was handled is noteworthy.

Not only did the unidentified witness appeal the lower court's ruling almost immediately, but the appeals court also responded quickly. The court set an accelerated briefing schedule, and all the judges involved moved quickly to amend a procedural flaw in just under a week. That typically would have taken "weeks or months" to resolve.

The unnamed witness in the subpoena case also took the unusual step of requesting a hearing before all 10 of the DC circuit court judges.

Gregory Katsas, one of the judges on the panel, recused himself from the matter. Katsas was appointed to the court by President Donald Trump. Prior to joining the panel, he worked in the White House counsel's office as deputy counsel to the president.

Getty Images/Chris Kleponis

An entire floor of a federal courthouse went into lockdown mode

The circuit court initially declined to hear the case, sending it back down to Howell, who oversaw another sealed hearing on October 5. CNN reported that Michael Dreeben, a veteran attorney working for Mueller's office who specializes in trial law, was in the hearing room when arguments took place.

The appellate court eventually decided to take up the case and set an argument date for December 14 before a three-judge panel.

• On December 14, as reporters milled around the hearing room on the fifth floor of the appeals courthouse where arguments were due to take place, building security put the floor on lockdown.
• CNN reported that security officers cleared the entire floor, including the courtroom - with the exception of a few law clerks who assist judges - the hallway, the vestibule, the stairwells, and the elevator banks.
• Officers reportedly also checked a coat closet on the floor to make sure no belongings had been left behind. At one point, the elevator didn't even open up to the fifth floor.

Over an hour passed with no sign of any attorneys entering or leaving the hearing room or the building, CNN reported. Then, about 10 minutes after activity at the courthouse appeared to wrap up for the day, reporters spotted a black car carrying Mueller team attorneys arrive back at the special counsel's office.

The unknown witness isn't a person - it's a foreign company

AP Photo/J. Scott Applewhite

Robert Mueller.

The mystery case took another interesting twist on Tuesday, when the appeals court issued a ruling which revealed the unnamed witness in the case isn't an individual, but a foreign corporation, described in documents as a company "owned by Country A."

The ruling indicates that the company has been fighting a subpoena from Mueller's office to hand over information to the grand jury on the grounds that doing so would violate the law in "Country A."

The document says the court rejected the company's claim that complying with Mueller's subpoena would break the law in the foreign country and ordered it to hand over the information prosecutors had demanded. It also revealed that the DC federal court is fining the corporation for every day that it doesn't comply with the special counsel's subpoena.

Read more: The 20 biggest Trump-Russia bombshells of 2018

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, told INSIDER the drip-drip nature of the case raises "many more questions" than answers.

A few things, however, can be gleaned from the case so far.

First, the foreign company has materials Mueller wants. Those could be in the form of emails or documents. The company also likely has a US presence, Cramer said, and the ability to garner support in "Country A."

"Prosecutors normally gain documents from a foreign entity via a Mutual Lateral Assistance Treaty request," he added. "That could have been requested here and resulted in a court fight. If there was no US presence, the foreign country could have simply denied the request."

Moreover, it's unusual that a government regulator from "Country A" supported the corporation's legal argument, as indicated in the appellate court's ruling. Regardless, the ruling indicates that the company will now have to comply with Mueller's subpoena.

"At the end of the day, these items will become known to a grand jury," and, potentially, the American public, depending on how they're used, Cramer said.